1. Field of the Invention
This invention relates to an improved optical information recording medium for use in optical recording systems.
2. Description of the Prior Art
As the recording layers used for the optical information recording medium, it has been proposed to use the thin films of organic dyes instead of those of metals such as Al, Te or the like. The thin organic dye films are of the advantages that they are low in melting point, decomposition temperature and heat conductivity, thus providing high densification and sensitivity; they can be formed by coating, thus achieving high mass-productivity at low cost; and they have thermal expansion, water permeability as well as other physical and chemical characteristics which are closed to those of plastic materials used as a substrate, thus reducing crack occurrence of the recording film and deformation by buckling of the plastic substrate.
When the thin organic dye films are formed on the plastic substrate by coating method, however, the problem remains that the plastic substrate is dissolved upon coating by a solvent having a high solubility to plastics, such as acetone, dichloroethane, which are often used as a coating solvent. If the plastic substrate is dissolved even slightly upon coating, the reflectance of the dye film is not increased, thus impairing the advantages of the thin organic dye films are previously mentioned. To overcome the above problem, it can be devised that coating is carried out by using a solvent which has no effect on the substrate. However, there are no dye materials having good storage life which are suitable for coating. Alternatively, there has been proposed a method of providing the recording layer of organic dyes on the substrate with which an underlayer has been provided such as a photopolymer layer. However, the problem remains that additional step of providing the underlayer brings increased cost and reduced productivity of the recording medium.
Further, commercially available injection molded plastic substrate for an optical disc have poor solvent resistance, and only water and alcohols have been used as the solvent which has no effect on the plastic substrate. Moreover, a resistance to environment (including temperature-humidity resistance, storage life) is strictly required for optical recording media, and therefore recording materials must be insoluble in water. From this point of view, there is a pressing need for a recording material which has high solubility in alcoholic solvents, good water resistance and high storage stability (light resistance, heat resistance), when a recording film is formed by coating on the injection molded plastic substrate which is contributive to cost down of the recording medium.
Therefore, it is an object of this invention to provide an optical information recording medium which has high C/N ratio, good storage life and low degradation on read-out and can be produced at low cost. Another object of the invention is to provide a dye compound being soluble in a solvent (e.g., water, alcoholic solvent and a mixed solvent thereof) which has no effect on a plastic substrate. Other objects of the invention will be apparent from the following descriptions.